A Comparative Evaluation of Four Acoustic Hydrophones in High Intensity Focused Ultrasound (HIFU) Field Measurements

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1 A Comparative Evaluation of Four Acoustic Hydrophones in High Intensity Focused Ultrasound (HIFU) Field Measurements Yunbo Liu and Keith Wear Food and Drug Administration, Silver Spring, Maryland

HIFU Therapy 0-20 Pressure Distribution 1~5

pressure Non-invasive focused ultrasound

0.4 0.2 0 X-axis (mm) Bone Metastases -0.2-0.

2 HIFU Therapy 0-20 Pressure Distribution 1~5 MHz Transducer Precise focus with high pressure Non-invasive focused ultrasound therapy Thermal ablation and cavitation damages Small focus with high pressures Patient safety & device performance Pre-clinical pressure field testing FDA approvals and clinical trials PII X-axis (mm) Bone Metastases Z-axis (mm) Uterine Fibroids Insightec Ltd MRI guidance/thermometry

HIFU Hydrophone Challenges Hydrophone in a strong, nonlinear and focusing HIFU field Hydrophone is a mechanical sensing device to convert acoustic pressure into electrical RF signals.

3 HIFU Hydrophone Challenges Hydrophone in a strong, nonlinear and focusing HIFU field Hydrophone is a mechanical sensing device to convert acoustic pressure into electrical RF signals. Hydrophone pressure measurement is recommended by IEC ultrasound standards and FDA regulatory guidance. Mechanical/cavitation damages How high a pressure can we measure? Hydrophone sensitivity 1. Bandwidth, frequency/directional response 2. Deconvolution of the nonlinear signals 3. Spatial averaging of the sensing element Measurement Reliability 1. Single hydrophone repeatability 2. Inter-hydrophone comparability Commercial Hydrophones the measurement opportunities are considerably restricted because of the HIFU fields by Bessonova and Wilkens, IEEE-UFFC, 2013

3-MHz single element spherical transducer

4 Materials Methods Lateral beam profiles Focal waveforms with increasing pressures 3.3-MHz single element spherical transducer with 2 x10 mm focus 3D scanning water tank with angular controls Four different hydrophones 1. Robustness 2. Sensing element size 3. Fiber Optic vs. Conventional 4. Sensitivity and deconvolution HIFU Transducer Onda Corp., US Piezoceramic needle um PVDF Capsule um Fiber optic 1 bare tip Fiber optic 2 -- Fabry Perot Sonic Concepts H101 RP Acoustic, GER 100 um 10 um fiber PA Corp., UK

5 Driving levels Comparison Methods Transducer driving/outputs Driving voltage Acoustic Power (W) Level Level Level Level Amplitude (V) Hydrophone voltage linear vs nonlinear Time(us) Measurement repeatability Needle hydrophone Hydrophone voltage V+ (mv) V- (mv) Vrms (mv) March 2012 test June 2013 test June 2014 test Average 72.7 ± ± ± 2.64 Coefficient of Variation % 3.26% 3.95% 5.49%

6 Pressure distribution Needle hydrophone Normalized Pressure Normalized Pressure Same beam distribution with increasing 3.3 MHz HIFU output. Lateral Distance (mm) Level 1 Level 4 Level 8 level 1 Level 4 Level Z axis (mm)

7 Lateral Pressure Distribution Normalized Pressure Needle Capsule Fiber Optic-1 Fiber Optic MHz 1.00 HIFU 2.00 Lateral distance (mm) Normalized Pressure (db) Lateral distance (mm) Spatial Averaging Effect (IEC ) Hydrophone radius wavelength/2.5 (almost satisfied) Pressure hydrophone radius 1dB decrease from peak (satisfied)

Complex Deconvolution pressure(t) Pressure(f) Hydrophone M(f) voltage(t) Voltage(f) Pressure(f) = Voltage(f) / M(f) IEC 62127-1 Hydrophone

8 Complex Deconvolution pressure(t) Pressure(f) Hydrophone M(f) voltage(t) Voltage(f) Pressure(f) = Voltage(f) / M(f) IEC Hydrophone deconvolution Frequency Domain Time Domain pressure(t) = -1 [Pressure(f)] = -1 [Voltage(f) / M(f)] Complex operation -----Original Deconvolution

9 Pressure comparison- waveform Fiber Optics 1 level1 level2 20 level3 level4 16 level5 level6 level7 12 level8 Pressure (MPa) Time (um)

10 Pressure (Mpa) Pressure comparison Output level 1 Capsule Needle FO-1 FO Time (us) Pressure (Mpa) Pressure (Mpa) Output level Time (μs) Needle FO-1 Capsule Needle FO Output level 8 Time (us)

11 Pressure comparison 3.3 MHz outputs P+, Compressional pressure (MPa) P-, Rarefactional pressure (MPa) Capsule Needle FO-1 FO-2 Variation % Capsule Needle FO-1 FO-2 Variation % Level % % Level % % Level % % Level % % Level % % Level % % Level % % Level % % Medium nonlinearity was presented at high output levels. Similar measurement variation across most output levels. Rarefactional pressure P- varied more than the compressional pressure P+ (15% vs 7%).

Nonlinear Propagation Parameters (local distortion parameter) Zp is the axial distance of the point of interest to the source aperture plane; Pm is the mean-peak-cycle acoustic pressure β is the

12 Nonlinear Propagation Parameters (local distortion parameter) Zp is the axial distance of the point of interest to the source aperture plane; Pm is the mean-peak-cycle acoustic pressure β is the nonlinearity parameter (β = 3,5 for water at 20 C); ω is the angular frequency; Fa is the local area factor (beam shape). * IEC (2014): Specification and measurement of field parameters for high intensity therapeutic ultrasound (HITU) transducers and systems Nonlinear Propagation 3.3 MHz outputs Capsule Needle FO-1 FO-2 Level Level Level Level Level Level Level Level

13 Pressure (Mpa) F F T Output level 8 Time (us) Needle FO-1 How nonlinear? Nonlinearity Needle Hydrophone: σ q = 2.0 FO-1 Hydrophone: σ q = 1.7 According to IEC (HITU Measurement Standard), σ m < 0.5 is considered quasi-linear condition and applicable to this standard. O FO Needle O FO Needle 10

14 Spatial Averaging Correction IEC Specification and measurement of field parameters for high intensity therapeutic ultrasound (HITU) transducers and systems When the hydrophone is translated from the position of maximum received signal in any direction normal to the beam axis by an amount equal to the effective hydrophone radius element, the decrease in signal should be less than 1 db. If this is not the case, corrections for spatial averaging should be made. See Annex E of IEC :2007 The correction factor, Ksa, at the center of the beam is: Ksa = (3 β)/2=1.125 Where β = 0.75 (signal at one hydrophone radius from the axis)/(signal on axis) 3.3 MHz P+ (MPa) P_ (MPa) Needle Original Corrected Original Corrected Level Level Level Level Level Level Level Level

15 HIFU Hydrophone Test Summary HIFU beam profile and pressure amplitude comparison by 4 medical ultrasound hydrophones Good agreement for P+ and P- among hydrophones Hydrophone calibration accuracy spatial averaging signal deconvolution and nonlinearity electronics/system/operator repeatability IEC (New HITU hydrophone standard) does not cover high amplitude focused field measurement. Higher frequency measurement is underway. The mention of commercial products, their sources, or their use in connection with material reported herein is not to be construed as either an actual or implied endorsement of such products by the FDA.

High Intensity Focused Ultrasound Test & Measurement Techniques

High Intensity Focused Ultrasound Test & Measurement Techniques Shmuel Ben-Ezra June 2011 What's HIFU??? 2 What's HIFU??? High Intensity Focused Ultrasound Plays an increasing role in medical-therapeutic